Coastal and regional marine heatwaves and cold-spells in the Northeast Atlantic

Abstract. The latest IPCC report describes an increase in the number and intensity of marine heatwaves (MHWs) and a decrease in marine cold-spells (MCSs) in the global ocean. However, these reported changes are not uniform on a regional to local basis and it remains unknown if coastal areas follow the open ocean trends. Ocean temperature measurements collected by satellites (from 1982–2022) and 13 coastal buoys (from 1990–2022) are analyzed in the Northeast Atlantic and three subregions: English Channel, Bay of Brest and Bay of Biscay. The activity metric, combining the number of events, intensity, duration and spatial extent, is used to evaluate the magnitude of these extreme events. The results from in situ and satellite datasets for each of the studied regions are quite in agreement, although the satellite dataset underestimates the amplitude of activity for both MHW and MCS. This supports the applicability of the method to both in situ and satellite data, albeit with caution on the amplitude of these events. Also, this localized study in European coastal Northeast Atlantic water highlights that similar changes are being seen in coastal and open oceans regarding extreme events of temperature, with MHWs being more frequent, longer, and extending over larger areas, while the opposite is seen for MCSs. These trends are explained by changes in both the mean and variance of sea-surface temperature. Besides, the pace of evolution and dynamics of marine extreme events differs among the subregions. The English Channel is the region experiencing the most drastic changes over the last four decades. Summer marine heatwaves were very active in the English Channel in 2022 due to long events, in the Bay of Biscay in 2018 due to intense events and in the Bay of Brest in 2017 due to a high occurrence of events. Winter MCSs were the largest in 1987 and 1986 due to long and intense events in the English Channel. Finally, our findings suggest that at an interannual time scale, having dominant high-pressure conditions over Northern Europe and a low off the Iberian Peninsula favor the generation of strong summer MHWs in the Northeast Atlantic, while the opposite situation in winter dominates for MCSs.